Energy Internet is a future development trend of the electric power industry. With rapid development of large-scale centralized new energy power generation, distributed power generation, power grid frequency modulation services, and micro power grid technology, there is a great demand for energy storage equipment. The addition of energy storage equipment in a power system can suppress fluctuation of power grid brought by the connection of large-scale new energy power generation to the power grid, turn a “rigid” power system into a “flexible” power system, and improve safety, economical efficiency, and flexibility of the power system during operation.
The energy storage equipment has dual attributes of “generation” and “load”, and a converter for connecting the energy storage equipment and the power grid also needs to have a bidirectional operation function. Currently, there are mainly two types of bidirectional isolation circuits: bidirectional resonant CLLC circuit and bidirectional active bridge circuit.
The bidirectional resonant CLLC circuit has advantages such as high efficiency, high power density, and wide voltage input range. However, the traditional method for controlling the bidirectional resonant CLLC circuit is frequency conversion control. When an input voltage and an output voltage are the same, the larger a switching frequency of the bidirectional resonant CLLC circuit is, the smaller the power transmitted by the circuit is; and when the power transmitted by the circuit is zero, the switching frequency is infinite theoretically. Since an operation switch frequency of a power device is limited, the power transmitted by a circuit cannot be reduced to a small extent. As a result, smooth switching of an operation direction of the bidirectional resonant CLLC circuit cannot be achieved only by the traditional frequency conversion control.
Although the bidirectional active bridge circuit has problems of large off-current and circulating current, it can achieve smooth switching of an operation direction. At present, on occasions where fast and stable switching of the operation direction is required, such as scheduling of energy storage equipment, V2G technology of electric vehicles, and energy feedback, only the bidirectional active bridge circuit can be used. Application of a traditional bidirectional resonant CLLC circuit is restricted on these occasions by its shortcoming of not being able to achieve smooth switching of an operation direction.
Therefore, it is desirable to make improvements to existing technologies.